Joomla 2.5 user permissions ( ACL ) are a bit tricky to understand and get to grips with for the first time. They offer an incredible level of control to a large website and bring joomla to a truly enterprise level for intra and extra nets. They also lead to confusion if they are not setup correctly.

The Permissions control are not hierarchical.

It can be made hierarchical if you use inherit permission.

There is no limit on the groups and access levels. You have users, user groups and access levels. Groups are assigned to access levels

A user can be part of many user groups.

Access levels control the permissions to access categories, articles, menus or components

Access levels have user groups assigned to them.

A user group has 9 available actions

Site Login – login to the front - end site.

Admin Login - login to the backend administrator site.

Super Admin – Same as joomla 15 , can do any thing on the site front and back-end.